Wrap connection.commit with OpenTracing

This is an attempt to diagnose poor apdex levels, per
https://github.com/matrix-org/internal-config/issues/1181

synapse/storage/database.py

@@ -50,6 +50,7 @@ from synapse.logging.context import (
     current_context,
     make_deferred_yieldable,
 )
+from synapse.logging.opentracing import trace
 from synapse.metrics import register_threadpool
 from synapse.metrics.background_process_metrics import run_as_background_process
 from synapse.storage.background_updates import BackgroundUpdater
@@ -104,8 +105,20 @@ def make_pool(
         # Ensure we have a logging context so we can correctly track queries,
         # etc.
         with LoggingContext("db.on_new_connection"):
+            # HACK Patch the connection's commit function so that we can see
+            #      how long it's taking from Jaeger.
+            class NastyConnectionWrapper:
+                def __init__(self, connection):
+                    self._connection = connection
+                    self.commit = trace(connection.commit, "db.conn.commit")
+
+                def __getattr__(self, item):
+                    return getattr(self._connection, item)
+
             engine.on_new_connection(
-                LoggingDatabaseConnection(conn, engine, "on_new_connection")
+                LoggingDatabaseConnection(
+                    NastyConnectionWrapper(conn), engine, "on_new_connection"
+                )
             )
 
     connection_pool = adbapi.ConnectionPool(